I'm using Big Sur and renamed my admin account. Now it has been downgraded to a standard account. There are other admin accounts on the system, so backing everything up and reinstalling wouldn't work. I can't use single user mode because that doesn't exist anymore. Removing .AppleSetupDone, for some weird reason, does not work either.
How else should I try to regain my admin privileges?
Hopefully the absent admin get back to you sooner than you decide to wipe the Mac and start over.
In the mean time, you can try the normal “forgot” steps in case you remember or have access to any alternate unlock methods to override an admin account.
My hunch is you’re trying to remove a non-existent .AppleSetupDone file from the data volume and didn’t realize or accommodate for the Big Sur system volume being immutable / sealed.
A possible solution is to boot from a recovery drive (like a USB stick), using any UN*X system (Linux, MacOS, *BSD, etc). However, I recommend a MacOS installer if your drive is encrypted by FileVault.
So after you have gained have full access to the drive (eh, you are root :), you just have to create a new user account with full privileges.
However, be careful: this is not a trivial thing to do, and you better have a sysadmin at hand for helping you through it.
